The present invention generally relates to a process of producing magnetic recording media and to products produced thereby. More particularly, the present invention relates to metal plating of zinc plated aluminum and, more specifically, to an improved process of treating a substrate aluminum to provide an exceptional surface condition prior to further plating processes. The present invention is particularly advantageous in both alkaline and acid double zinc plating processes.
Magnetic recording impulse memory devices are extensively used in computer and data processing systems. These devices generally include a substrate on which a thin film of magnetic material has been deposited. The present invention has particular advantageous application to the manufacture of rigid memory disks having large storage capacities.
The requirements of the magnetic recording media of these disks are critical since the reading and writing of information data on the surfaces thereof are usually performed by magnetic transducers which are supported in flying heads positioned in closely spaced relation to the surfaces of the disk. Aluminum is the preferred substrate for the disks although other suitable metals may be employed. In general, a relatively thin layer of nonmagnetic electroless nickel is applied on the aluminum followed by a thin layer of magnetic material such as cobalt. A signal is stored on the disk by magnetizing the cobalt layer to represent the signal at a selected moment in time.
Typical alloys used for memory disks are Aluminum Association Nos. 5086 and 5586. Rigid memory disks have, in the past, commonly been manufactured by initially cleaning the surface of an aluminum disk which renders it receptive to a non-magnetic coating, electrolessly depositing a non-magnetic nickel containing layer thereon, cleaning and activating, or cleaning, activating and mechanically polishing that non-magnetic layer to make it receptive to a magnetic layer, and then depositing a magnetic electroless cobalt containing layer thereon. Thereafter, the disk can be cleaned and a lubricant or other protective coating may be applied over the magnetic layer.
U.S. Pat. No. 4,659,605 discloses a process for producing a high density magnetic storage device such as a rigid memory disk having improved magnetic recording properties and comprising the steps of cleaning the surface of a substrate to provide a substantially oil and oxide free substrate surface, electrodepositing a non-magnetic nickel-phosphorus layer on the substrate and, electrolessly depositing a magnetic cobalt-phosphorus layer consisting essentially of cobalt and phosphorus on the nickel-phosphorus layer wherein the electroless cobalt-phosphorus deposition is conducted in an ammonia-free bath containing a source of cobalt ions, a source of hypophosphite ions, and a complexor constituent which includes conjointly, an effective amount of each of a source of citrate ions and a low molecular weight, bath soluble amino acid. The '605 patent also discloses a process wherein the cleaned substrate is zinc plated prior to the deposition of the non-magnetic nickel-phosphorus layer. The '605 patent further discloses the zinc coated substrate can be subjected to a zinc stripping operation by immersion of the disk in a suitable acid such as, for example, nitric acid, followed by a further zinc plating step.
U.S. Pat. No. 5,141,778 discloses a method of preparing aluminum memory disks using a double zinc plating procedure wherein a nitric acid bath initially consisting essentially of nitric acid and a specified added amount of a Group VIII ion is used to treat the first zinc coated substrate and the zinc coated aluminum substrate is plated using an electroless metal plating bath containing an effective amount of cadmium ions. As noted by the '778 patent, aluminum alloys when exposed to the air rapidly forms an oxide which can present plating problems. As a result, special treatments must be employed when plating on aluminum. These treatments generally include mechanical treatments; chemical etches; alkaline displacement solutions; and anodizing.
Despite the existence of methods for treating aluminum for subsequent plating the need still exists for an improved process of preparing an aluminum substrate for memory disk applications. In this regard, although nitric acid baths have been used to prepare the aluminum substrate for subsequent plating and to remove the first layer of zinc deposited thereon in double zinc plating processes, the presence of nitrate ions has been found in certain applications to limit the effectiveness and degree of depositing zinc upon the substrate. The surface condition of the zinc coated aluminum can affect the surface condition of the non-magnetic nickel layer deposited thereon. The degree of subsequent polishing of the nickel coated layer depends upon the condition of the nickel coat. Accordingly, it is desirable to have the best possible zinc coated aluminum substrate prior to nickel deposition.